The present invention relates generally to the monitoring of neuromuscular function in humans, and more specifically to an apparatus and method for monitoring neuromuscular blockage during induction of anesthesia prior to an operation, during the operation itself, during the reversal of the neuromuscular block and after the operation is complete.
During the past four decades, muscle relaxants have been used routinely during anesthesia to facilitate endotracheal intubation, to simplify the control of artificial respiration, and to make intra-abdominal and thoracic operations possible. More recently, the use of muscle relaxants in intensive care units has gained widespread acceptance for intubation, to permit decreased ventilator pressures and to reduce patient oxygen consumption.
A significant drawback to the use of neuromuscular blocking agents is the wide variation and lack of predictability of patient response thereto. Even in healthy patients, there are large individual differences in sensitivity, which may be aggravated by disease (myasthenia gravis), hypothermia, disturbed acid-base balance and altered liver and kidney function. Suffice it to say, patient response is unpredictable to the extent that clinical evaluation thereof has proved to be inadequate in a large number of instances.
The most serious side effect of an overdose of muscle relaxant is post operative respiratory failure, generally termed residual curarization by the medical community. After anesthesia has ended, the patient may not be able to breathe because of weak respiratory muscle strength. This hazard alone has called for a means to effectively assess neuromuscular blockade to that muscle relaxant overdose can be avoided, as the assumption that relaxation has been reversed and that a patient is capable of self-sustaining respiration can only be ascertained when neuromuscular function is accurately measured.
Muscle relaxation is commonly measured during induction of anesthesia, during the operation, during the reversal of the neuromuscular block and after completion of the operation. Generally, aside from clinical observation by the anesthesiologist of unstimulated muscle movement, a technique which is highly subjective and virtually impossible with unconscious patients, electrical nerve stimulation is currently the preferred monitoring technique. One prior art apparatus for providing such stimulation is disclosed in U.S. Pat. No. 4,157,087.
A nerve stimulator is attached to a motor nerve of the patient before induction of anesthesia, and is switched on after the patient is asleep. An electrical stimulation current is then applied to the nerve, and those muscles supplied by the nerve will contract. There is maximum contraction of a muscle fiber if stimulus intensity exceeds a certain threshold. The incremental increase in the force of muscle contraction is graded by and proportional to the number of muscle fibers activated. If the motor nerve is stimulated sufficiently, as by a current of adequate magnitude, all of the muscle fibers supplied by the nerve will contract and the maximum force of contraction is obtained. Further increase in stimulus intensity, or application of a supramaximal stimulus, does not increase contraction force. It is desirable to increase the electrical stimulus gradually until the supramaximal level is reached, to provide a baseline response. However, many practitioners do not utilize supramaximal stimuli due to the attendant acute patient discomfort. It is also possible for the practitioner to utilize a lesser level of electrical stimulation, and to follow the trend of neuromuscular blockage as anesthesia is induced.
Once the maximum muscle reaction is known to a reference stimulation, a myoneural blocking drug is injected. In most situations, a drug dose which produces a depression of 90-95% of muscle response, generally termed 90-95% "twitch" depression, is adequate to ensure sufficient muscle relaxation while facilitating antagonism of the postoperative block.
Various techniques have been employed in the prior art to monitor patients' muscular reactions to electrical nerve stimuli. The first, visual or tactile observation of muscle movement following electrical stimulation, has been shown to be inadequate in identifying fade is muscle response with sufficient accuracy to exclude residual curarization.
The second, use of a force transducer to measure strength of muscle response to stimulation of the ulnar nerve, requires that the patient's hand and thumb be restrained and connected to a force transducer and monitor. Such devices are disclosed in U.S. Pat. No. 4,387,723 and 4,848,359, and in "Self-Tuning, Microprocessor-based Closed-loop Control of Atracurium-induced Neuromuscular Blockade" by P. C. Uys et al, British Journal of Anesthesiology (1988), 61, pp. 685-692 and "New Developments in Clinical Monitoring of Neuromuscular Transmission: Measuring the Mechanical Response" by J. Viby-Mogensen et al, pp. 56-59. Aside from being both cumbersome and costly to implement, the method may result in nerve or tissue damage if the thumb is not positioned properly with respect to the transducer linkage, due to the necessarily rigid connection thereto.
A third technique, evoked electromyography, or EMG, records the compound action potential caused by stimulation of a peripheral nerve. EMG devices and methods are disclosed in U.S. Pat. No. 4,291,705 and 4,595,018, and in "Computer-Controlled Muscle Paralysis with Atracurium in the Sheep" by D. G. Lampard et al, Anaesthesia and Intensive Care, Vol. 14, No. 1, (1986) pp. 7-11; "Clinical automatic control of neuromuscular blockade", by A. J. Asbury et al, Anesthesia, Vol. 41 (1986) pp. 316-320; "Closed-loop administration of atracurium" by N. R. Webster et al, Anesthesia Vol. 42 (1987) pp. 1085-1091; and "Measuring the compound EMG in the use of muscle relaxants" by J. F. Crul et al, Excerpta Medica (1983) pp. 60-65. EMG signal amplitude is typically in the range of 3 uV to 5,000 uV, the duration about 3 ms to 15 ms, and the frequency range 2 Hz to 10,000 Hz. While surface electrodes are conveniently employed to noninvasively record the overall electrical activity of the muscles, good electrical contact is difficult to maintain at the skin/electrode interface, and the system, due to the small magnitude and short duration of the signals, is difficult to isolate from electrical interference.
Yet another technique of the prior art employ a thumb-mounted miniature accelerometer to measure patient response to stimulation. The method is convenient, but the device is both expensive and fragile and measures movement in only one direction. Thus, accelerometer mounting orientation is critical, as the use of a triaxial accelerometer with attendant hardware and software to resolve the signals would be prohibitively expensive. U.S. Pat. No. 4,817,628 suggests the use of facial-mounted accelerometers, and acknowledges that the use of bi-or triaxial accelerometers may be necessary for meaningful data.
Finally, combinations of several of the above prior art methods and devices have been utilized, as disclosed in "A Comparison of Computer-controlled Versus Manual Administration of Vecuronium in Humans" by R. R. Jaklitsch et al, Journal of Clinical Monitoring, Vol. 3, No. 4 (1987) pp. 269-276, wherein both a thumb-mounted force transducer and EMG monitoring are employed.